This invention relates to a device and a process for the removal of volatile constituents from polymers, in particular for vaporising volatile components from polymer solutions by indirect heat exchange. The device has at least one container with an inlet for the polymer solution and outlet for the volatile components and an outflow for the polymer from which the volatile components have been removed, and a heat exchanger with a plurality of channels, which form a heat exchange zone, wherein the channels have a length of 1.0 to 40 cm, a height, constant over the length thereof, of 1.3 to 13 mm and a width of 1 to 10 cm in the entry zone of the channels and wherein the width of the channels at least doubles between the entry thereof and the exit thereof
The removal of volatile components from a polymer solution is one of the final processing stages in the production of many polymers. The volatile constituents to be removed may be either solvents or unpolymerised monomers. Depending upon the viscosity of the polymer solution, various methods are known for the removal of the volatile components from polymer solutions, each involving heating the polymer solution by means of a heat exchanger to a temperature above the vaporisation temperature of the volatile constituents. Drying devices which are described are, for example, film evaporators, extruders and those with indirect heat exchange.
It is vital during heating of the polymer solution that the polymer is not thermally degraded.
Published patent application EP-A-150225 describes an apparatus having two heat exchange bundles connected in series. The heat exchange bundles have rectangular channels. This apparatus is primarily used for two-stage heating or cooling during the reaction, but is a relatively complex piece of equipment. EP-B-226204 discloses a process and a heat exchanger for the removal of volatile constituents from a polymer solution containing at least 25 wt. % of polymer. The polymer solution is heated in an indirect heat exchange zone, which consists of a plurality of channels. The channels have a substantially uniform ratio of surface area to volume in the range from 0.158 to 1.97 mmxe2x88x921, a height of 1.27 to 12.7 mm, a width of 2.54 to 10.16 cm and a length of 1.27 to 30.48 cm. The polymer solution is heated in the channels at a pressure of 2-200 bar to a temperature above the vaporisation temperature of the volatile components, but below the boiling temperature of the polymer. The residence time of the polymer solution in the channels is 5 to 120 seconds. After heating, the solution is transferred into a chamber in which at least 25% of the volatile constituents are stripped from the solution. The process reduces thermal damage by reducing the time for which the polymer is exposed to elevated temperatures. One disadvantage of the process is, however, that it is not possible to achieve complete removal of the solvent in a single stage. Moreover, polymer deposits form on the outside of the heat exchange bundle which carbonise over time and occasionally flake off, so contaminating the polymer from which the solvent has been removed.
EP-B-352 727 discloses a process for the removal of volatile constituents from polymer solutions by heating the polymer solution to above the vaporisation temperature of the volatile components in a plurality of channels connected in parallel. The ratio of heat exchange surface area to the volumetric flow rate of the product is xcx9c80 m2/m3/h. The flow velocity in the channels is xcx9c0.5 mm/s and the residence time of the polymer solution in the channels is 120 to 200 seconds. This process also has the disadvantage that complete removal of the solvent is not possible in a single stage. Moreover, polymer deposits form on the outside of the heat exchange bundle which carbonise over time and occasionally flake off, so contaminating the polymer from which the solvent has been removed.
The object thus arises of providing an apparatus and a process for the removal of volatile components from a polymer solution which do not exhibit the disadvantages of the prior art.
The object is achieved by providing a device of the present invention for the removal of volatile components from a polymer solution comprising at least a container (30), which has an inlet (1) for the polymer solution, an outlet (3) for the volatile components and an outflow (2) for the polymer solution from which the volatile components have been removed, and a heat exchanger (31), which is arranged inside the container (30), with a central receiving zone (21) for the polymer solution, which is connected to the container inlet, a product zone (18) to receive the treated polymer solution, a heat exchange unit (31) having a plurality of channels (14), which form a heat exchange zone and which connect the receiving zone to the product zone (18), a heating means (13) to heat the heat exchange unit (31) and the channels, characterized in that the channels (14) have a length of 1.0 to 40 cm, a height, constant over the length thereof, of 1.3 to 13 mm and a width of 1 to 10 cm in the entry zone of the channels (14) in the receiving zone (21), wherein the width of the channels (14) at least doubles between the entry (23) thereof and exit (32) thereof to the product zone (18).
The present invention provides a device for the removal of volatile components from a polymer solution comprising at least one container, which has an inlet for the polymer solution, an outlet for the volatile components and an outflow for the polymer solution from which the volatile components have been removed, and a heat exchanger arranged inside the container which has a central receiving zone for the polymer solution, which is connected to the container inlet, a product zone to receive the treated polymer solution, a heat exchange unit having a plurality of channels, which form a heat exchange zone and which connect the receiving zone to the product zone, a heating means to heat the heat exchange unit and the channels, characterised in that the channels have a length of 1.0 to 40 cm, a height, constant over the length thereof, of 1.3 to 13 mm and a width of 1 to 10 cm in the entry zone of the channels in the receiving zone, wherein the width of the channels at least doubles between the entry thereof and the exit thereof to the product zone.
The channels preferably have a rectangular cross-section and the width of the channels at the outlet is three times as large as the width of the channels at the inlet, wherein the channels may widen continuously, but in accordance with any desired profile.
In a preferred embodiment, the channels widen parabolically.
A device in which the width of the channels is kept constant for at least half the length thereof and then widens to at least double the width, wherein the enlargement proceeds continuously, but in accordance with any desired, in particular non-linear, profile is likewise preferred. When an appropriately adjusted temperature is used, the channel shapes ensure that the width of the channel is constant in the zone in which the polymer solution is heated and does not enlarge until the polymer solution has reached a temperature of above the vaporisation temperature of the volatile components, such that these may readily be stripped out of the solution while still in the channels.
In one variant, both the width and the height of the channel may widen towards the exit in the preferred manner.
The heat exchanger of the device according to the invention preferably has at least 100 such channels. In particular, however, there are 200 to 100,000 channels in the heat exchanger.
In a preferred embodiment, the heat exchanger has a cylindrical shape, such that the channels surround the receiving zone in a cylindrical configuration. The heat exchanger is preferably arranged in the area of the channel outlets that the individual channels directly abut with each other laterally and/or at the upper and under side thereof, such that there are no zones between the channel exits on which the polymer material may be deposited. This applies to both the cylindrical and any other configuration of the channels.
In another preferred embodiment of the device, the heat exchange unit in particular has a cuboid shape and is arranged beneath the receiving zone.
In a preferred embodiment, the heat exchange unit is formed from a plurality of superposed or adjacent plate segments arranged in planes, wherein the plate segments are spaced apart in one plane, and wherein the spacing thereof and the lateral profile in the plane determine the width of the channels and the thickness of the plate segments determines the height of the channels.
In a preferred variant of the device, the heat exchange unit is formed from a plurality of superposed or adjacent plates arranged in planes, which plates are separated by spacers, wherein the spacing and lateral profile of the spacers determine the width of the channels and the thickness of the spacers determines the height of the channels.
The device is preferably entirely or in part, in particular those parts in contact with the polymer solution, made from a metallic material having a low iron content which contains at most 10 wt. %, preferably at most 5 wt. % of iron.
The material having a low iron content is preferably tantalum or a nickel alloy having a low iron content and is in particular selected from the range Alloy 59 (2.4605), Inconell 686 (2.4606), Alloy-B2, Alloy-B3, Alloy-B4, Hastelloy C-22, Hastelloy-C276, Hastelloy-C4, preferably Alloy 59.
The heat exchanger has any desired means known to the person skilled in the art for heating the channels to temperatures above the vaporisation temperature of the volatile components. These means are, for example, resistance heaters or a network of tubes to convey a heat exchange fluid.
The channels in the heat exchange unit are preferably inclined over the length thereof by a downwards angle relative to the horizontal towards the exit thereof, in particular, they are arranged vertically. In this case, the receiving zone is arranged above the channels.
The heating means for the heat exchanger preferably comprise a plurality of tubes which pass transversely relative to the channels through the plate segments or through the plates and through which a heat exchange fluid circulates.
The present invention also provides a process for the removal of volatile components from a polymer solution containing at least 40 wt. % of polymer using the device according to the invention, wherein the process comprises:
A) introduction of the polymer solution into the receiving zone at a pressure of 1 to 100 bar abs.,
B) passage and heating of the polymer solution in the channels of the heat exchanger to a temperature above the vaporisation temperature of the volatile components of the polymer solution and below the boiling or decomposition temperature of the polymer, wherein the residence time of the polymer solution in the channels is from 5 to 120 sec,
C) separation of the volatile components from the polymer solution through the outlet and
D) discharge of the polymer from which the volatile components have been removed.
The device and process according to the invention may be generally applied for the removal of volatile components from sensitive solids, in particular from thermoplastic polymers, elastomers, silicone polymers and high molecular weight lubricants and similar substances.
Preferably, however, the process according to the invention is used for degassing thermoplastic polymers. These polymers comprise any plastics which flow under the action of pressure and heat. Polystyrene, polycarbonate, polyphenylene, polyurethane, polyamide, polyester, polyacrylate, polymethacrylate may be mentioned by way of example in this connection. The process is very particularly suitable for degassing polycarbonate.
The volatile components may be both unpolymerised monomers and solvents. One solvent frequently used in the production of thermoplastic polymers is, for example, methylene chloride, or a mixture of methylene chloride and chlorobenzene.
The polymer solutions contain at least 40 wt. % of polymer. In the molten state, the polymer solution typically has a viscosity of 0.5 to 200 Pas.
In the process, the polymer solution is in particular compressed into the channels at a pressure of 1.5 to 50 bar abs., preferably of 2 to 5 bar abs., flows through the channels of the heat exchanger and is so heated to a temperature of preferably 250 to 350xc2x0 C. The pressure prevailing at the outlet of the channels is preferably below the saturation pressure of the volatile components at the particular temperature. The pressure in the product zone is preferably less than or equal to 105 Pa, in particular from 3000 Pa to 105 Pa.
Preferably, the pressures upstream and downstream from the channels, the temperature in the channels and the shape of the channels are selected such that the volatile components are already completely separated from the polymer in the channels.
The pressure in the receiving zone and the temperature in the channels is in particular selected such that at least 95%, in particular at least 98%, preferably at least 99.5%, particularly preferably at least 99.8% of the volatile components of the polymer solution vaporise in the channels.
The residence time of the polymer solution in the channels is typically 5 to 120 sec, preferably 80 to 120 sec. The flow velocity of the polymer solution is in particular from 0.0001 to 0.01 mm/s, preferably from 0.001 to 0.005 mm/s. The ratio of heat exchange surface area of the channels to the volumetric flow rate of the polymer solution is 5 to 75, preferably 15 to 50 m2/m3/h.